Authors of a new study suggest that 520-million-year-old structures, previously identified as the brains of ancient arthropods, are instead preserved microbial biofilms.

Unlike related species, Ovatiovermis cribratus, a lobopodian from the Cambrian period, did not have a hard, protective shell.

These millimeter-size sea creatures lived 540 million years ago.

Ocean animals have been getting bigger over the millennia, according to an analysis of thousands of genera that have plied Earth’s seas since the Cambrian Period.